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(CNN)Donald Trump has outlined a new screening process for would-be immigrants designed to exclude Islamic extremists that he promoted Monday as "extreme vetting." The policy would attempt to establish whether applicants' beliefs match US values on gay rights, gender equality and religious freedoms, among others.

With little in the way of specifics from the Republican presidential candidate and his senior campaign officials -- including whether they would apply to tourists -- the proposed vetting reforms have raised questions about how officials would assess the validity of applicants' responses and whether an overhaul of the immigration screening process would work.

As it stands now, the US already has an extenstive system for screening potential immigrants. Here are some key things to know about what the US already does and how Trump's plan would be different.

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"It's easy enough to say whatever you want to say on a questionnaire. Then the question becomes how much time and how intensely is someone going to be scrutinized -- what is that going to do to the overall immigration system in the United States?"

He added: "It does raise some serious concerns in terms of how one would implement such a policy."

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There are exceptions to the rules -- for refugee asylum or inter-country adoption, for example. There are also special categories that are largely employment-based and extend to former US government workers such as Iraqi or Afghan translators or contractors.

And then there are diversity visas, often referred to as the "green card lottery," which each year admit a limited number of people from countries "with historically low rates of immigration" to the US.

Foreign nationals' eligibility to apply for an immigrant visa must be confirmed by the US Citizenship and Immigration Services before their paperwork is passed to the National Visa Center.

The long path to approval

The National Visa Center then begins to process the application -- which can take up to three weeks -- before handing off documentation to the appropriate US embassy or consulate abroad. At the same time, applicants must pay processing fees and start to collate the visa application form and numerous required documents -- including financial records such as bank statements and civil records covering birth, marriage, criminal and military history among others.

Administrative processing' can take months or, in extreme cases, years.

Charlotte Slocombe, a partner at immigration law firm Fragomen

It can take as long as six weeks for the National Visa Center to complete its record check and issue an invitation for an interview. Generally these are scheduled within 60 days of complete document submission and one month in advance. Prior to their interviews, candidates must also undertake a full medical exam.

Interviews are conducted by a US consular officer abroad, during which time digital fingerprint scans are also taken. Multiple government agencies also conduct security screenings that generally takes one to two weeks, according to Charlotte Slocombe, a partner at immigration law firm Fragomen.

"(It) can take considerably longer for applicants of certain backgrounds, particularly applicants from countries known for terrorism or from professions dealing in sensitive technologies," she told CNN. "This 'administrative processing' can take months or, in extreme cases, years."

Trump's three-pronged approach

The new values test would be part of a three-pronged approach to Islamic extremism that Trump unveiled during a major policy address on Monday.

Trump's proposal starts with a ban that would "temporarily suspend immigration from some of the most dangerous and volatile regions of the world that have a history of exporting terrorism." This appears to be the candidate's latest version of his controversial blanket travel ban on all Muslims entering the US.

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Secondly, upon taking office, he would ask that the State Department and Department of Homeland Security provide him with a list of countries where adequate screening "cannot take place." Trump did not name any countries specifically, and it was not clear what he considers inadequate vetting.

Finally, he called for a more stringent immigration test to limit admission to the United State to only "those who share our values and respect our people."

He elaborated, "In the Cold War, we had an ideological screening test. The time is long overdue to develop a new screening test for the threats we face today. I call it extreme vetting. I call it extreme, extreme vetting."

How 'extreme vetting' would work

A Trump campaign official, in providing Afghanistan as an example of a country with many who hold inimical views, said that many individuals there "may have attitudes about women or attitudes about Christians or gays that would be considered oppressive, even violent."

The adviser continued, "We have no reason to bring someone into our country who is going to harbor that hostility. We want to bring in people who are reformers or who support moderation or who embrace or expand pluralistic ideas," the campaign adviser said.

Slocombe said Trump's proposal would require a wide broadening of the security review process and consequently put a severe strain on the State Department and other government agencies.

"It would require a near overhaul of the application form and very likely capture many more thousands of applicants in the 'administrative processing' net," explained Slocombe, describing a need for new application and interview standards -- likely causing significant delays "although arguably not adding any greater security checks than already exist."

She added, "US immigration is sometimes criticized for being too subjective, and Mr Trump's plans may lend greater credence to this charge."

"It does raise some serious concerns in terms of how one would implement such a policy," said Lanhee Chen, CNN political commentator and previously a public policy director for former Republican presidential candidate Mitt Romney.

"It's easy enough to say whatever you want to say on a questionnaire," he said. "Then the question becomes how much time and how intensely is someone going to be scrutinized -- what is that going to do to the overall immigration system in the United States?"

There are some historical precedents

Trump's proposal to screen the ideology of would-be immigrants is not completely without historical precedent.

The State Department's DS-260 form that all prospective immigrants must currently fill out asks screening questions based on actions, like whether the applicant is a member of a terrorist organization, rather than beliefs. But during the 20th Century, the US barred immigrants who believed in communism or anarchism.

After President William McKinley was assassinated by an anarchist in 1901, the US passed a law two years later banning anarchists from entry into the US, Philip E. Wolgin, managing director for immigration at the Center for American Progress Action Fund, told CNN.

Wolgin added that the US broadened the restrictions in 1952 to include those affiliated with the Communist Party and other political groups in favor of overthrowing the US government. In addition to some people being barred from coming to the US, others were deported once their affiliation with these political groups was discovered by US authorities.

These entry bans were eventually lightened or eliminated during the last years of the Cold War, and the current Immigration and Nationality Act states that no one will be barred from the US for "past, current, or expected beliefs, statements, or associations, if such beliefs, statements, or associations would be lawful within the United States."

But to this day, US immigration form DS-260 still asks would-be immigrants whether they are a "member of or affiliated with the Communist or other totalitarian party."